Method and device for converting heat into mechanical work

ABSTRACT

A novel method converts heat into mechanical work. In a cyclic process, a working medium is compressed while giving off heat and it is subsequently brought in thermal contact with the surroundings via a first heat exchanger. Then it is expanded while obtaining mechanical work, whereupon the cyclic process is run through once more. A high degree of efficiency is achieved by virtue of the fact that the working medium, after expansion, is guided through another heat exchanger, which is situated inside a rapidly rotating rotor and which, on the exterior thereof, is surrounded by at least one substantially annular gas space from whose exterior heat is dissipated. There is also disclosed a device for carrying out the novel method.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for converting heat into mechanicalwork, in which a working medium is compressed in a cyclic process whilegiving off heat and is subsequently brought in thermal contact with theambient environment via a first heat exchanger, is then expanded whileobtaining mechanical work, whereupon the cyclic process is run throughagain.

2. Description of the Related Art

Numerous working methods are known to convert thermal energy intomechanical work. Usually, a working medium is compressed, heated,expanded in the heated state and cooled in such cyclic processes,whereupon the cyclic process starts again. The precondition for suchcyclic processes is that two different temperature levels are availablewhich are used for heating or cooling the working medium. Generally, acertain temperature is defined as the ambient temperature, which is thetemperature of a medium which is available in an unlimited andgratuitous way. This can be the air temperature of the ambientenvironment for example or the temperature of a water body from whichwater can be taken in sufficient quantities for purposes of temperatureexchange.

No cyclic processes are known with which it is possible to gainmechanical work from thermal energy without disposing over a heattransfer medium whose temperature differs substantially from ambienttemperature. According to current substantially from ambienttemperature. According to current belief such a cyclic process isexcluded by the second law of thermodynamics. It is stated in a moreprecise version of the second law of thermodynamics that the efficiencyof any cyclic process for converting thermal energy into mechanical workcannot exceed the so-called Carnot efficiency which is calculated fromthe ratio of the available temperature levels. Real existing methods andapparatuses are generally also far away from the Carnot efficiency.

Apparatuses for generating temperature differences are known which usegas-dynamic effects occurring at high accelerations in order to producetemperature differences. These apparatuses are not suitable in order toperform cyclic processes for gaining mechanical work.

DE 38 12 928 A shows an apparatus which tries to overcome the abovedisadvantages. Even with such an apparatus it is not possible to improvethe efficiency to a relevant extent.

It is the object of the present invention to provide a method of thekind mentioned above which allows obtaining mechanical work from thermalenergy with the highest possible efficiency.

It is a further object of the invention to provide an apparatus withwhich the method in accordance with the invention can be performed.

SUMMARY OF THE INVENTION

In accordance with the invention, this method is characterized in thatthe working medium, after expansion, is guided through another heatexchanger which is situated inside a rapidly rotating rotor and which,on the exterior thereof, is surrounded by at least one essentiallyannular gas chamber from whose exterior heat is dissipated.

The inventor of the present invention has recognized that by includingthe static gas theory in connection with considering gravity oracceleration acting upon the gas molecules or atoms it is possible toillustrate cyclic processes which have an especially high efficiency.The problematic aspect in this connection is however that the effectscaused by gravity are very small, as a result of which technicalimplementation is very difficult. As a result of the cyclic process inaccordance with the invention, the utilization of thermal energy forgenerating mechanical work can be achieved under economically viableframework conditions. A substantial precondition for the method inaccordance with the invention is the achievement of the highestaccelerations by a rapidly running rotor, with the achieved accelerationvalues being chosen as high as possible.

It is especially preferable when the working medium is guided downstreamof the rotor through a compressor. The heating caused in the compressoris so low in any case that the working medium cooled in the rotorremains beneath the ambient temperature. This ensures that the workingmedium takes up ambient heat in the first heat exchanger.

In an especially advantageous embodiment of the method in accordancewith the invention it is provided that the working medium is guided inthe axial direction through the rotor. The effects of high accelerationin the interior of the rotor on the pressure conditions in the workingmedium can be eliminated substantially.

The present invention further relates to an apparatus for withdrawingheat at ambient temperatures with a rotor having a heat exchanger whichcan be flowed through substantially in the axial direction and which isdelimited on its outside by a cylindrical wall on the outside of whichthere is provided at least one substantially annular gas chamber.

This apparatus is characterized in accordance with the invention in sucha way that the heat exchanger is provided with a substantiallyring-cylindrical configuration and that the gas chamber is subdivided inthe radial direction into several ring-cylindrical partial chambers.These partial chambers can have the same dimensioning in the radialdirection, but can also be provided with different configurations. Onlythe described configuration of the rotor allows realizing a cyclicprocess of the kind mentioned above in a technically and economicallyviable manner.

It is principally possible that in each of the individual partialchambers the same gas is present. In such a case, the pressure on theoutside of a partial chamber is generally higher than the pressure onthe inner side of the further partial chamber adjacent to said partialchamber. This means that although the pressure increases from the insideto the outside as a result of centripetal acceleration within theindividual partial chambers, this increase is interrupted at theboundaries of the individual partial chambers. This leads to amechanical loading of the separating walls between the individualpartial chambers. This is technically controllable because the resultingpressure force acts towards the outside and the separating walls are notloaded to bulging. Preferably however, different gases are received inthe different partial chambers which especially have different criticaltemperatures and pressures. It can thus be achieved that the pressureload of the separating walls is minimized because in the balanced statesubstantially the same pressure applies inside and outside. It is alsowithin the scope of the present invention that gas mixtures are usedinstead of pure gases, which gas mixtures form concentration gradientsduring the operation of the apparatus.

As a result of the extremely rapid rotation of the rotor, the pressurepresent in the interior of the rotor differ in the idle statesubstantially from those in the operating state. In order to minimizethe loading of the separating walls and the other components, it isprovided in an especially preferred embodiment of the invention that apressure control device is provided which is in connection with thering-cylindrical partial chambers in order to set the internal pressure.In an especially preferred manner, the ring-cylindrical partial chambersare separated from each other by thin-walled cylindrical separatingwalls. Mechanical loads of the individual components can thus beminimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be explained below in closer detail byreference to the embodiments shown in the drawings, wherein:

FIG. 1 shows a schematic view of an apparatus for performing the methodin accordance with the invention;

FIG. 2 shows a rotor of the apparatus of FIG. 1 on an enlarged scale;

FIG. 3 shows a sectional view along line III-III in FIG. 2;

FIG. 4 shows a diagram illustrating the temperature curve in the radialdirection of the rotor, and

FIG. 5 shows a Ts-diagram explaining the cyclic process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The apparatus of FIG. 1 consists of a turbine 11 for the expansion ofthe working medium, which turbine is divided into two sections 11 a, 11b. A heat exchanger 11 c is provided in the first section 11 a in orderto enable an isothermal expansion. It is principally possible to provideseveral turbine stages in which the working medium is expanded in anadiabatic way and the heat exchangers are provided between the turbinestages, as a result of which only an approximately isothermal expansionis achieved. If the heat exchanger 11 c and the turbine 11 are providedthemselves, then it is actually possible to achieve a substantialisothermal expansion. The adiabatic expansion occurs in the secondsection 11 b of the turbine 11. The cooling medium is therefore presentat the output of the turbine 11 with a temperature which lies beneaththe ambient temperature.

A generator 12 is driven by the turbine 11 and a rotor 13 of acentrifuge is simultaneously driven which is flowed through by theworking medium in the axial direction. Compression occurs in acompressor 14, whereupon the working medium is guided back to theturbine 11 again via a recirculation line 15.

Rotor 13 comprises a ring-cylindrical heat exchanger 18 and several gaschambers 17 a, 17 b, 17 c, 17 d which are also provided with aring-cylindrical configuration and lie outside of the heat exchanger 18.Notice must be taken that the dimensions of the heat exchanger 18 andthe gas chambers 17 a, 17 b, 17 c, 17 d in the radial direction areshown on an excessive scale in FIG. 1, because in the case of realconfigurations these dimensions are very small and the heat exchanger 18and the gas chambers 17 a, 17 b, 17 c, 17 d lie close to the outerjacket of the rotor 13. Rotor 13 is provided on its outer side withcooling ribs 19 which represent a heat exchanger for dissipating heat.This is indicated by arrows 20.

The gas chambers 17 a, 17 b, 17 c, 17 d are preferably filled withdifferent gases, with the innermost gas chamber 17 a being filled withhelium for example, the adjacent gas chamber 17 b with xenon, the thirdgas chamber 17 c with nitrogen or a suitable hydrocarbon, and theoutermost gas chamber 17 d with a suitable coolant. As a result of therapid rotation of the rotor 13, a temperature drop from the outside tothe inside is caused in the gas chambers 17 a, 17 b, 17 c, 17 d whichstrongly cools the working medium in the heat exchanger 17.

Heat at the temperature level of the ambient environment is supplied tothe heat exchanger 16, which is indicated by arrow 21. An increase inthe efficiency can be achieved when the waste heat of the rotor 13 isalso supplied to the heat exchanger 16 according to arrows 20.

FIG. 2 shows the rotor 13 in detail in a modified embodiment. Theworking medium is supplied in the interior of a hollow-bored first shaft22 which is held in a bearing 23 and is guided via distributor lines 24to the outside radially to the heat exchanger 18. In the interior of theheat exchanger 18 the working medium flows in the axial direction to theopposite side of rotor 13 in order to be guided in further distributorlines 25 radially to the inside to a further shaft 26 held in a bearing27. As in the preceding embodiment, the four gas chambers 17 a, 17 b, 17c, 17 d are provided radially inside one another. A heat exchanger 18 isprovided on the outside for dissipating heat. A housing 28 is indicatedin a schematic manner, in which the rotor is arranged in a rotatable waywhich comprises a plurality of magnets 29. The magnets 29 are used forrelieving the bearings 23 and 27 at high speeds and are in interactionwith magnets (not shown) on the outside of rotor 13 itself. The polarityis directed in such a way that the magnets 29 and the magnets on rotor13 repulse one another, as a result of which an inwardly facing force isexerted on the jacket surface of the rotor 13 which considerably reducesmechanical stress as a result of centrifugal forces and allows higherspeeds. At least one gas container 30 is provided in the interior of therotor 13, which gas container is in connection with one of the gaschambers 17 a, 17 b, 17 c, 17 d via lines (not shown). Preferablyhowever, the compensating reservoir 30 comprises sub-containers (notshown) which are individually connected with the individual gas chambers17 a, 17 b, 17 c, 17 d. The mean pressure level in the gas chambers 17a, 17 b, 17 c, 17 d can thus kept at a predetermined value irrespectiveof the respective speed of the rotor 13, so that mechanical loading ofthe separating walls between heat exchanger 18 and the gas chambers 17a, 17 b, 17 c, 17 d remains within predetermined limits.

The following tables 1 to 4 show by way of an embodiment the statevariables of the gases or gas in the individual gas chambers 17 a, 17 b,17 c, 17 d, with table 1 relating to the innermost gas chamber 17 a,table 2 to the gas chamber 17 b, table 3 to the gas chamber 17 c andtable 4 to the gas chamber 17 d. The left half of the table indicatesthe state variables on the outside wall of the respective gas chamber 17a, 17 b, 17 c, 17 d and the right half of the table indicates therespective state variables on the inner wall of the respective gaschamber 17 a, 17 b, 17 c, 17 d.

The references mean the following in the tables 1 to 4:

T Temperature in K d Density in kg/m³ p Pressure in MPa s Entropy inkJ/kgK u Inner energy in kJ/kg h Enthalpy in kJ/kg

TABLE 1 T 276.32 T 121.51 d 174.43 d 28.62 p 14.33 p 0.91 s 5.18 s 5.18u 173.15 u 81.95 h 255.33 h 114.07

TABLE 2 T 424.17 T 276.32 d 129.39 d 50.25 p 17.61 p 4.07 s 5.62 s 5.62u 294.47 u 195.45 h 430.58 h 276.45

TABLE 3 T 579.04 T 424.17 d 94.29 d 45.76 p 17.54 p 5.88 s 5.98 s 5.98 u419.52 u 307.62 h 605.58 h 436.27

TABLE 4 T 739.98 T 579.04 d 77.64 d 42.67 p 18.39 p 7.54 s 6.24 s 6.24 u550.60 u 426.66 h 787.48 h 604.32

FIG. 3 schematically shows a sectional view along line III-III in FIG.2, with the heat exchanger 18 and the cooling ribs 19 having beenomitted for improving clarity of the illustration. Arrows 20 symbolizethe heat flow.

FIG. 4 shows a diagram which schematically states the temperaturedistribution in the radial direction of the rotor 13 which is stated byr. The curve K₁ represents the temperature T in the idle state, i.e.when no heat flow occurs, which is the case when the rotor 13 isinsulated on the inside and outside. Curve K₂ represents the temperatureT in operation, i.e. when there is a heat flow in the radial direction.

FIG. 5 shows an idealized T/s diagram, in which the temperature isentered over the entropy. The cyclic process is passed in the directionof the arrows 31. The double arrow 32 shows the temperature differenceof the centrifuge, i.e. the rotor 13 over the gas chambers 17 a, 17 b,17 c, 17 d. As a result of the losses in the heat transfer, thetemperature difference 33 which can actually be used in the cyclicprocess is considerably lower. The states 1, 2, 3, 4 in the diagramcorrespond to the states in the analogously designated points in FIG. 1.It can be noted for example that in the case of a single-phase workingmedium the changes in state 1→2 and 3→4 are not precisely isothermal.

[K] [kg/m³] [MPa] [kJ/kgK] kJ/kg kJ/kg T d P s u h Point 1 130 150.54937258 5.44088686 92.1986033 128.823442 Point 2 130 70 2.102576624.92707388 77.8876766 107.924486 Point 3 283 316.5007 30.24865724.92707388 153.810311 249.382476 Point 4 283 92.150807 7.660413465.44088686 192.911843 276.040941

The present invention allows realizing an apparatus and a cyclic processwith efficiencies which substantially exceed those of conventionalsolutions.

1. A method of converting heat into mechanical work, in a cyclic processcomprising the following steps: compressing a working medium whilegiving off heat; subsequently bringing the working medium into thermalcontact with an ambient environment through a first heat exchanger;expanding the working medium and thereby obtaining mechanical work;guiding the working medium through a second heat exchanger disposedinside a rapidly rotating rotor, the rotor including at least onesubstantially annular gas chamber surrounding the second heat exchanger;and radially dissipating heat away from the second heat exchanger,through the annular gas chamber and away from an exterior of the gaschamber.
 2. The method according to claim 1, which comprises guiding theworking medium through a compressor downstream of the rotor, in aworking medium flow direction.
 3. The method according to claim 1,wherein the working medium takes up ambient heat in the first heatexchanger.
 4. The method according to claim 1, which comprisesconducting the working medium through the rotor substantially in anaxial direction thereof.
 5. The method according to claim 1, wherein atemperature difference is built up in the rotor of at least 100 K. 6.The method according to claim 5, wherein a temperature difference isbuilt up in the rotor of at least 300 K.
 7. The method according toclaim 6, wherein a temperature difference is built up in the rotor of atleast 500 K.
 8. The method according to claim 1, which comprisesdissipating the heat via cooling ribs on an outside of the rotor.
 9. Themethod according to claim 1, which comprises dissipating the heatthrough a third heat exchanger on an outside of the rotor.
 10. Anapparatus for converting heat into mechanical work, comprising: a devicefor compressing a working medium; a turbine configured to expand theworking medium to obtain mechanical work, said turbine including a firstheat exchanger configured to obtain the working medium from the deviceand to subsequently bring the working medium into thermal contact withan ambient environment; and a rotor having an axis defining an axialdirection; said rotor including a second heat exchanger disposedtherein; said second heat exchanger configured to conduct the workingmedium substantially in the axial direction, having a substantiallyring-cylindrical configuration, and being outwardly bounded by asubstantially cylindrical wall; said rotor including a substantiallyannular gas chamber divided, in a radial direction, into a plurality ofring-cylindrical partial chambers; said annular gas chamber configuredto radially conduct heat away from said second heat exchanger; and saidsecond heat exchanger being surrounded by said annular gas chamber. 11.The apparatus according to claim 10, wherein said partial chambers areconfigured to receive mutually different gases.
 12. The apparatusaccording to claim 11, which comprises a pressure control devicecommunicating with said ring-cylindrical partial chambers for setting aninternal pressure therein.
 13. The apparatus according to claim 12,wherein said pressure control device is disposed in a region of saidaxis of said rotor.
 14. The apparatus according to claim 10, whichcomprises cylindrical separating walls separating said ring-cylindricalpartial chambers from one another.
 15. The apparatus according to claim10, wherein the working medium is fed in and discharged, respectively,through shafts of said rotor.
 16. The apparatus according to claim 10,which comprises a housing with magnets disposed to hold said rotor insaid housing by exerting an inwardly directed magnetic force on acircumference of said rotor.
 17. The apparatus according to claim 10,wherein said gas chamber is subdivided in the radial direction into atleast three ring-cylindrical partial chambers.
 18. The apparatusaccording to claim 10, wherein said gas chamber is subdivided in theradial direction into at least four ring-cylindrical partial chambers.19. The apparatus according to claim 10, wherein said first heatexchanger is configured to isothermally expand the working medium. 20.The method according to claim 1, which comprises isothermally expandingthe working medium while performing the step of subsequently bringingthe working medium into thermal contact with the ambient environment.